Dynamic Branched Fractures in Pulverized Rocks from a Deep Borehole

Korngreen, Dorit; Sagy, Amir

Detection of paleo-catastrophic earthquake and distinguishing between faults that generated earthquakes to more "quite" faults were always been challenging tasks for geoscientists. Recently, zones of pulverized rocks were found in outcrops near major faults in California and Japan and it was hypotheses that they are products of massive dynamic fragmentation occur during mega-earthquakes in the upper part of the earth crust. However, weathering and other near-surfaces processes suspected been involved with the formation of the pulverization.

New supporting evidence for the origin of the pulverization by catastrophic events was discovered in rocks from ~5 km depth where a deep fault zone had been drilled through. A unique pattern of micro-fractures developed in well compacted, homogenous, and fine grained carbonates were recovered, characterized by on-going hierarchical branching of tight tensile cracks. The unique branching is a well known effect of instability in rapid tensile fractures when approaching elastic wave velocities. These fractures that have direct evidences of been created in deep burial setting, demonstrate that the pulverization is indeed an effect of dynamic loading and fracturing. Their extending to a significant depth indicates that a large part of earthquake energy can be absorbed by off-fault deformation.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013